In this kind of engine, for example, Patent Document 1 discloses a homogeneous charge compressed ignition engine, wherein a mixture gas in which fuel and air are previously mixed is supplied to a combustion chamber in a cylinder to compress the mixture gas, thereby self-igniting the mixture gas. Patent Document 1 also discloses that a spark ignition operation (SI operation) is performed in engine starting in which a compression ignition operation is hardly performed.
The homogeneous charge compressed ignition engine has an advantage that thermal efficiency is high because operation can be performed at a compression ratio higher than that of a spark ignition type engine. Generation of NOx can also be restrained because a combustion temperature can be lowered. However, in the homogeneous charge compressed ignition engine, because the mixture gas is naturally ignited, it is extremely difficult to control ignition timing.
The compression ignition of the mixture gas heavily depends on an engine torque and an intake air temperature of the mixture gas. For example, FIG. 3 is a graph showing operation ranges where the spark ignition operation and the compression ignition operation can be performed in terms of a relationship between the engine torque and the intake air temperature of the mixture gas, the numeral Z1 designates an operation range where the general spark ignition operation (SI operation) can be performed, and the numeral Z3 designates an operation range where the compression ignition operation (HCCI operation) can be performed.
In FIG. 3, in order that the engine is started by the spark ignition operation to make a transition from the spark ignition operation to the compression ignition operation, it is necessary to raise the intake air temperature as shown by an arrow Y1. When an intake air temperature (T2) is raised to the operation range Z3 where the compression ignition operation can be performed, a load can be fed to increase the torque (power). However, in order to increase the torque, the compression ignition operation can be maintained as long as the intake air temperature is lowered again as shown by an arrow Y2. In order that the load is cut off to decrease the torque after the maximum load is applied, the compression ignition operation can be maintained as long as the intake air temperature is raised again.
Accordingly, it takes a long time to lower or raise the intake air temperature until the load is fed or the load is completely cut off since the intake air temperature is raised, which results in a problem in practical use.
In the conventional homogeneous charge compressed ignition engine, a fuel control valve (air/fuel valve) is adjusted according to the load while a throttle valve fully opened, and an engine speed is governed by changing a fuel concentration. Therefore, in the case where the feed or cutoff of the load is performed at the intake air temperature set as shown by the arrow Y3 of FIG. 3, the fuel concentration becomes weak during the low load in the governing in which the fuel control valve is used, which results in a problem in that self-ignition is not generated. Alternatively, there is a method in which the engine speed is governed by adjusting the throttle valve while the fuel control valve is kept constant. However, in such cases, the throttle is opened near a fully open state when the load is increased, and a mixture gas amount is hardly changed even if the throttle opening is varied, which results in a problem in that a speed control insufficiently functions.
Usually an exhaust gas catalyst is provided in an exhaust pipe of the engine in order to purify a hazardous component (air-pollution substance) contained in the exhaust gas. As can be seen from FIG. 12(B), the air-pollution substance (for example, CO) contained in the exhaust gas is properly purified by passing through the catalyst when the load is increased, while the air-pollution substance is not purified too much when the load is decreased. This is attributed to the fact that, as shown in FIG. 12(A), an exhaust gas temperature is lowered to insufficiently promote a chemical reaction for purifying the air-pollution substance when the load is decreased. Accordingly, it is necessary that the exhaust gas temperature be kept at a high level to properly purify the air-pollution substance even if the load is decreased.
Patent Document 1: Japanese Patent Publication Laid-Open No. 2005-69097